A key element of micro conduit systems for e.g. biochemical analysis is flow control. The flow control typically involves flow inducing means; such as e.g. external pumps for pumping either liquid or air into the micro conduit system. Alternatively, the flow inducing means may provide centrifugal force acting on a liquid contained by the micro conduit system, and/or capillary forces acting on the liquid.
Another key element of flow control is valving, i.e. the control of fluid flow within the micro conduit system by opening and/or closing valves. Throughout the last 15 years numerous schemes for microfluidic valving have been proposed and tested by researchers. The early attempts on devising microfluidic valves often involved moving parts, e.g. embodied in check or gasket valves micro-machined in silicon and often involving a number of different materials.
Work on simpler and more rugged valving systems (see e.g. US2006036348A1) has produced valves without moving parts or with e.g. solidified wax comprising the “moving part”. Michigan-based HandyLab has been a leader in developing microfluidic valves based on wax acting as a blocking member, respectively being molten and thus opened on demand by activating microscale ohmic heating elements.
The capillary valve and the closely related hydrophobic valve, collectively referred to as capillary-stop valves, are well-established valve types that have no moving parts and are easily implemented in the production of a micro conduit system. They function by introducing an abrupt increase in the effective contact angle between the liquid to be controlled and the inner surface of the micro conduit system, e.g. by means of change of geometry or a change of the wettabililty of the inner surface of the micro conduit system. A capillary-stop valve may for example comprise a sudden increase of channel diameter with a close to 90 degrees transition from a small to a larger diameter or it may comprise a substantially hydrophobic section or a combination of the two.